Wire-cut discharge machines cut a workpiece in the manner such that: a wire electrode is fed from a supply reel, and moved between a pair of electrode positioning guides under a predetermined tension, and sent and collected by an apparatus such as a take-up reel or the like; the workpiece approaches the wire electrode from the direction which is almost perpendicular to the axis of the wire electrode and forms a work gap; kerosene, demineralized water or the like is supplied as a working fluid in this gap; a train of machining voltage pulses is supplied between the wire electrode and the workpiece, thereby generating a train of pulse discharges in the work gap, while the workpiece and wire electrode are relatively fed.
However, in the conventional wire-cut discharge machine, there is a problem that accidental breaking of the wire electrode occurs.
If the electric power supplied to the wire electrode is limited to a low level, the incidence of accidents can be held to an extremely low level.
However, by doing so, the machining speed decreases and also in certain cases, the objects of machining are never realized.
The breaking of wires occurs at various locations for various reasons.
It has been conventionally considered that more of such accidents are caused when the gaseous discharge or other abnormal discharge is generated at the discharge point in the work gap.
Therefore, conventionally, in order to reduce the incidence of accidental breakage and to safely and continuously perform the machining by use of high current at high speed and under high load, it has been considered that it is sufficient to supply a large amount of working fluid to the discharge point so as not to cause the gaseous discharge in the working gap; thus, for attainment of such object, engineers have been mainly concerned with improvements in the technology of supplying the working fluid to the discharge point in the working gap.
However, the inventors of the present invention have discovered that although accidental wire breakage occurs due to the abnormal discharge which is generated at the discharge point, in many cases, it is caused by spark discharges which occur at the contact point of the wire electrode and the current supplying apparatus for supplying a train of current pulses for machining, under certain working conditions or circumstances, and that this problem will never be solved by only the improvement in the above-mentioned working fluid supplying technology, and according to this discovery, the inventors completed the present invention.
The current supplying apparatus according to the conventional technology is constituted such that no harmful gaseous discharge is generated between the wire electrode and the current supplying apparatus in the manner such that: for instance, the central axis of the apparatus is disposed perpendicularly to the passage of the wire electrode; the apparatus has a cylindrical surface arranged so that the wire electrode can come into contact therewith with a strong surface pressure; and the above-mentioned cylindrical surface portion where the current supplying apparatus which comes into contact with wire electrode is surrounded and cooled by the cooling water or the like, which is discharged along the wire electrode, or if not so, it is disposed in the main body of the working fluid jet nozzle filled with the working fluid, or another but similar method.
Such a cylindrical current supplying surface is widely used since it has the advantages that: the processing of it, particularly, the abrasive finish is very easy; both the friction and contact resistance are small; it also is useful to straighten the wire electrode; the desired contact area and contact surface pressure can be substantially obtained; and due to this, it is possible to feed a current even to the thin wire electrode having a diameter of about 0.05 to 0.3 mm.
Although it was conventionally known that it is important to sufficiently cool such a current supplying apparatus, it was not known that even if this cooling is sufficiently performed, spark discharges occur and these discharges will cause accidental breaking of the wire electrode at the contact portion of the current supplying apparatus and the wire electrode.
In addition, it was not therefore understood that the shape of the current supplying apparatus is also important to solve such a problem.
By improvement of this portion according to the present invention, the incidence of accidental breaking of the wire electrode can be remarkably reduced and this makes it possible to carry on machining for a long time using a high current pulse train under high load.